1. Field of the Invention
The present invention relates to the preparation of selectively permeable pores in separation membranes of all kinds.
2. Background of the Invention
The use of membrane filters to separate molecules and particles by size is well-known, but passage of molecules or particles larger than the nominal pore size, in a membrane or container wall, is not well developed in the prior art.
In many instances, controlled passage through pores can be accomplished by occluding the pores with an erodible material to provide a separation barrier with plugged pores such as is disclosed in U.S. Pat. Nos. 5,026,342 and No. 5,261,870, incorporated herein by reference. With the separation membrane disclosed in these patents, transmembrane passage is allowed only after appropriate environmental conditions have caused or allowed erosion of the material plugging the pores. Mechanical control devices are thus unnecessary, and such membranes are useful to initiate controlled passage of materials of appropriate size through the pores of the membrane.
The membranes in question, which contain pores for particular separations, take many forms. Certain membranes contain fibril membranes with a xe2x80x9chaystackxe2x80x9d structure, in which limits to a separation are accomplished by tortuous passageways of varying size through a crisscrossed bed of filter materials. Alternative membranes contain a sponge-like structure, in which tortuous pores of varying size and configuration are present, sometimes generated by proprietary technologies involving gas extrusion or other pore generation techniques. Additional membrane types may have capillary-pore or track-etched membranes with a xe2x80x9ctunnelxe2x80x9d structure, created in a solid sheet of base membrane material by inducing highly controlled physical damage in the membrane and then etching the damaged areas to create pores of uniform diameter through the membrane. Various polymers of many types may be used to form all of these and other membranes.
Information from published product literature illustrates another difference between microporous and capillary-pore membranes. Literature produced by Millipore, Inc. provides illustrations and notes that in their microporous membranes the passageways have a range of diameters. It is stated that their xe2x80x9c10 micrometer porexe2x80x9d product has approximately 68% passageways of nominal 10 micrometer diameter (range not known), but approximately 32% passageways smaller than the nominal 10 micrometers. In contrast, one vendor of capillary-pore membranes (Oxyphen AG) provides illustrations and states that the tunnel-like pores are of uniform diameter and all are within xc2x110% of the stated diameter. Hence, capillary-pore membranes have greater uniformity of the transmembrane passageway than microporous or fibril membranes do.
To increase the value of any membrane filter, the surface of the membrane and/or the passageways can be changed after formation of the pores, by one of several chemical modifications. One example is plasma treatment of the membrane in the presence of ammonia gas to create amine functions on exposed surfaces, or by activation followed by a plunge into acrylic acid. Such treatments change transport properties across the passageways or pores. Other examples of providing membranes responsive to environmental changes include preparation of: 1) co-polypeptide membranes leading to a membrane with microdomains of polyamino acids and, thereby, providing a responsive base membrane; and 2) asymmetric polypeptide membranes, with a two layered membrane providing a responsive network.
Inherent to the manufacture of capillary-pore membranes is extensive introduction of charges on the face of the tunnel or passageway, with minimal formation of charges on the general polymer faces. This is because capillary-pore membranes are produced by physically damaging polymer film in a controlled manner with a beam of heavy ions (e.g., krypton) in a cyclotron. The ions follow a linear path where interaction with polymer chains of the membrane releases energy to damage molecules in the polymer matrix. Damage represents latent pores, which subsequently are opened by chemical etching, such as with cyclical treatment with alkaline and acid solutions. Specifically, carboxyl functions may be formed with etching treatments such as these.
Literature on track etching and production and use of capillary-pore membranes reveals that it is known to track-etch thin sheets of solids; that coating pores with fatty acid monolayers or non-specific adsorbed proteins can be used to change pore properties; and that certain polymers such as cellulose polymers are very sensitive to environmental conditions so that quality of track images in cellulose polymers depends on how the material was prepared. In addition, it is also known that in order to create carboxyl functionality during track-etching, oxygen must be present, and in instances in which chemically active and/or ionic groups are desired to be attached to pores in order to mediate pore permeability, tunnels in capillary-pore membranes are well suited to promote attachment of such chemically active and/or ionic groups.
All of the above-described technology pertains in one way or another to the designing of permeable membranes of specialized design. Many membranes simply must be provided with initially completely plugged pores, however, when a separation membrane needs first to function as the wall of a container and to perform its separation function at a point later in time. Some of the disadvantages of erodible plugs for pores, in membranes and other separation devices, is that the erosion or hydrolysis products of the erodible plugging materials impart unwanted properties to the surrounding system, which system is in many cases a sensitive biological system. Also, erodibility of certain plugging materials is not as exacting as a particular application would dictate, with certain cellulose polymers being erodible but not as predictably or quickly as desired. A need remains, therefore, for a pore-plugging material, for separation barriers generally and membranes in particular, in which erosion of the plug after formation is precipitated by a particular pH event and in which the hydrolytic products of the erosion are biologically benign.
In order to meet this need, the present invention is a pore plugging material, for pH dependent membrane diffusion, in which cyclic olefins having phosphazene-functional moieties provide predictable erosion properties when used to plug pores is separation barriers and other porous membranes. Specific properties of the polymers are dependent on several factors, including molecular weight and identity of side groups attached to the phosphazene moiety. However, as a class, phosphazene-functional cyclic olefins provide both predictable erodibility and uniformly benign hydrolysis products and are, therefore, uniquely suitable as pore plugging polymers for separation barriers and membranes of all kinds. The invention, therefore, embraces the provision of a pH-sensitive erodible pore plugging material to pores in separation barriers and membranes of all kinds.